Widely tunable lasers are of great interest for both long-haul and metropolitan optical networks. Besides their use for source sparing with the advantages of reduced inventory and cost, they open the possibility of new system architectures with more efficient and more flexible network management. For example, the combination of tunable lasers with wavelength routers can provide large format-independent space switches and reconfigurable optical add/drop functions. High speed wavelength switching can also be used for wavelength based packet switching and optical CDMA.
Monolithically integrated semiconductor tunable lasers offer many advantages over external-cavity tunable lasers assembled from discrete components. They are compact, low-cost, and more reliable as they contain no moving parts. A conventional monolithic tunable laser usually comprises a multi-electrode structure for continuous tuning. A typical semiconductor tunable laser consists of a distributed Bragg reflector (DBR) grating, an active gain section, and a phase shift region. An electrode for electrical control is disposed on top of each of the three sections. When the reflection peak wavelength of the DBR grating is tuned by injecting current, the phase shift region must be adjusted simultaneously in order to prevent the laser from hopping from one mode to another. Besides, the tuning range of such a laser is limited to about 10 nm due to the limitation of commonly achievable refractive index change in semiconductor materials.
A more sophisticated tunable laser with a wider tuning range and improved performances was described by V. Jarayman, Z. M. Chuang, and L. A. Coldren, in an article entitled “Theory, design, and performance of extended tuning range semiconductor lasers with sampled gratings”, IEEE J. Quantum Electron. Vol. 29, pp. 1824-1834, 1993. It comprises of four electrodes controlling two sampled grating distributed Bragg reflectors, a phase-shift region and a gain section. Similar structures have been developed in the form of superstructure grating (SSG) DBR laser, modulated grating Y-branch (MG-Y) laser and digital supermode (DS) DBR laser, as described, respectively, in Y. Tohmori, Y. Yoshikuni, H. Ishii, “Broad-range wavelength-tunable superstructure grating (SSG) DBR lasers,” IEEE J. Quantum Electron. 29, 1817-1823 (1993); J.-O. Wesström, Gert Sarlet, Stefan Hammerfeldt, “State of the art performance of widely tunable modulated grating Y-branch lasers,” Optical Fiber Communication Conference, Washington D. C., paper TuE2 (2004); and D. J. Robbins, G. Busico, L. Ponnampalam, J. P. Duck, P. J. Williams, R. A. Griffin, A. J. Ward, D. C. J. Reid, N. D. Whitbread, and E. Barton, “A high power, broadband tunable laser module based on a DS-DBR laser with integrated SOA,” Optical Fiber Communication Conference, Washington D.C., Paper TuE3 (2004). In all these prior-art devices, the wavelength tuning requires complex electronic circuits with multidimensional current control algorithms and look-up tables. Such complexity reduces the fabrication yield and increases the cost, and also opens the questions about the manufacturability and long term stability of the devices.
A widely tunable or wavelength switchable laser can also be realized by using two coupled cavities of slightly different lengths. The coupled-cavity laser can be fabricated either by etching a groove inside a cleaved Fabry-Perot laser, as described in a paper entitled “Monolithic two-section GaInAsP/InP active-optical-resonator devices formed by reactive-ion-etching”, by L. A. Coldren et al, Appl. Phys. Lett., vol. 38, pp. 315˜317, 1981, or by using a cleaved-coupled-cavity, as described in a paper entitled “The cleaved-coupled-cavity (C3) laser”, by W. T. Tsang, Semiconductors and Semimetals, vol. 22, p. 257, 1985. However, the performance of the prior-art coupled-cavity lasers in terms of mode selectivity is not satisfactory, which results in very limited use for practical applications.
Coupled-cavity lasers have also been investigated in the form of a Y-laser, as described in an article entitled “The Y-laser: A Multifunctional Device for Optical Communication Systems and Switching Networks”, O. Hildebrand, M. Schilling, D. Baums, W. Idler, K. Dutting, G. Laube, and K. Wunstel, Journal of Lightwave Technology, vol. 1, no. 2, pp. 2066-2074, 1993, and the references therein. The Y-laser has the advantage of being monolithic without the challenging fabrication requirement for deeply and vertically etched trenches. However, the mode selectivity of the Y-laser is very poor, with a side-mode threshold difference of only about 1 cm−1 for a 450 μm laser, compared to over 10 cm−1 for a typical DFB laser. This is far from sufficient for stable single-mode operation.
It has also been proposed to use two passive ring resonators coupled to the waveguide in a Fabry-Perot laser for achieving single-mode widely tunable laser, as described in an article entitled “Passive Microring-Resonator-Coupled Lasers”, by B. Liu, A. Shakouri, and J. E. Bowers, Appl. Phys. Lett., vol. 79, pp. 3561-3563, 2001, and in “A GaInAsP-InP Double-Ring Resonator Coupled Laser”, by D. G. Rabus, Z. Bian, and A. Shakouri, IEEE Photonics Technology Letters, vol. 17, no. 9, pp. 1770-1772, 2005. These prior-art devices require an active Fabry-Perot cavity with cleaved facets, while the two ring resonators are used as passive transmissive filters inside the Fabry-Perot cavity to select the lasing mode. The device has a large size due to numerous elements and there is complexity due to the requirement that the filter transmission peaks of both ring resonators need to be tuned to match the Fabry-Perot mode for optimum performance.
For many applications, it is not necessary to tune the laser wavelength continuously. Rather, it is only required that the laser can be set to any discrete wavelength channel, e.g. as defined by the ITU (International Telecommunication Union). Such applications include linecard sparing, wavelength routing and optical add/drop. Key requirements for such wavelength switchable lasers are: 1) an accurate match of the discrete operating wavelengths with the predefined wavelength channels (e.g. ITU grid); 2) simple and reliable control for the switching between various channels; 3) high side-mode suppression ratio and low crosstalk; 4) fast switching speed; and 5) easy to fabricate and low cost.
In co-pending U.S. patent application Ser. No. 11/306,520 entitled “V-coupled-cavity semiconductor laser”, a V-coupled cavity structure with an optimal coupling coefficient for high single-mode selectivity is disclosed for operation as a widely wavelength switchable laser. The present patent application discloses a half-wave coupled active double ring resonator structure which can achieve the same functionality and performance without deep-etched reflection mirrors or any additional Fabry-Perot cavity.